While much has been done in the prior art to produce a fully suspended bicycle with attention given to isolating pedaling or braking forces from the suspension, the study of wheel path would appear to be focused on isolating these forces. For many implementations of the fully suspended bicycle, pedaling efficiency is of utmost concern. For what is known as a down hill mountain bicycle, or DH Bike, maintaining speed and control over extreme irregularities in the running surface is often the primary concern.
A DH Bike is almost always ridden in a standing position using gravity as the primary force for acceleration. Pedaling becomes a secondary method of maintaining the bicycle's velocity over rough terrain or to add additional acceleration when the terrain is not steep enough.
After several years of studying the prior art as a rider and technician, I find that the best suspension for a DH Bike is different than much of the prior art, not in pedaling or braking effects, but in terms of wheel path. This invention produces a wheel path that I consider most desirable in a way that corrects design issues of the few examples of the prior art that produce a similar wheel path.
When the prior art is arranged as shown in FIG. 2 through strictly symbolic representation we see that the top most row represents the wheel paths of the single pivot suspension: a low single pivot (21), a mid height pivot (22), a high single pivot (18) and a very high single pivot demonstrated by U.S. Pat. No. 606,323 June 1898 Wronski (23). The wheel paths of linkages are demonstrated in the second row as U.S. Pat. No. 5,121,937 June 1992 Lawwill (24), U.S. Pat. No. 5,509,679 April 1996 Horst Leitner (25), U.S. Pat. No. 7,128,329 Weagle (26) and U.S. Pat. No. 4,671,525 June 1987 Ribi (27). The third row represents Canadian Patent application #2357167 September 2001 Duval (26), U.S. Pat. No. 6,206,397 March 2001 Klassen et al (29), a superposition of the wheel paths demonstrated above from a common starting point (20) to their respective end points and in the lower right hand corner, this invention.
The wheel path of a low single pivot (21) arcs forward. A mid height single pivot (22) and Duval (28) ultimately move forward of their starting point, while Lawwill (24), Weagle (26) and Leitner (25), in ideal implementations, maintain a near vertical wheel path. It isn't until we see the ideal representation of the Klassen et al (29) wheel path that a slightly rearward path becomes apparent. The high single pivot (18) shows a much more dramatically rearward arcing path. The very high single pivot of Wronski (23) and the linkage system of Ribi (27) manage to produce a sufficiently similar and dramatically rearward arcing wheel path that they are shown on the same line as the representative wheel path of this invention (15).
The reason for pursuing this wheel path involves a short discussion on vector physics. As the rear wheel of a vehicle such as a bicycle encounters a positive variation in the running surface such as a bump or other obstacle at low speeds the impulse vector has a substantial vertical upward component and less of a horizontal rearward component. As speed increases the rearward component of the impulse vector increases. The path for the wheel or contact mounting point to travel to best absorb this impulse vector is in an increasingly rearward direction. Suspension does not only allow for the absorption of positive variations in the running surface, it also allows the vehicle to maintain an “in contact” condition through allowing the wheel to move downward from a typical running “sag” point to maintain contact with the running surface in the case of a dip or depression. Having a contact mounting point move in a forward direction when the suspension extends from its sag point allows for a faster return to a “in contact” condition given the forward direction of the vehicle.
The inventions of Wronski and Ribi both produce a similar wheel path but introduce certain design issues. Wronski requires a very high mounting point of the vehicle's frame to produce a dramatically rearward arcing path and the use of a concentrically mounted jackshaft to route the drive chain to the pivot to avoid issues of changing the tension on the drive chain during suspension activation. Ribi introduces linkages low on the vehicle frame, and in the case of U.S. Pat. No. 5,452,910 September 1995 Harris, a bicycle specific invention that produces this rearward wheel path, we find that not only are the pivots low on the frame of the vehicle, the cranks move with the suspension and change their distance from the handle bars and, less importantly for the purposes of a DH bike, from the saddle of the bicycle.
With the wheel path as the primary focus of this invention the issues of pedaling and braking effects must be addressed to ensure that additional problems are not introduced.
The traditionally held negative effect of pedaling on a fully suspended bicycle is known as pedal bob. This is often attributed to rotational forces about the rear wheel or to drive chain tension activating the suspension independently of any need to absorb irregularities in the running surface. Activation of the suspension from pedaling can also be observed to be induced by the unbalanced momentum of a downward pedal stroke. While an ideal pedal stroke would provide smooth power to the drive train throughout the entire crank rotation this is not necessarily possible or practical from a standing position and in situations where surface irregularities make a smooth pedal stroke difficult from clearance issues alone.
The traditionally held view of braking effects on a vehicle's suspension, where that vehicle is a bicycle, discuss issues of rotational forces and an unbalanced pull on the frame from the location of pivots. While this is often studied from the perspective of the vehicle alone, it should be noted that a typical rider weighs 75kg (165lb) and a typical DH Bike weighs 18kg (40lb). The mass of the rider must be added to the calculations of vehicle momentum and the resultant centre of gravity must be seen as that of the rider and vehicle. Considering that a standing riding position is typical for a DH bike, the centre of gravity of the bicycle and rider combined is thus above that of the vehicle; under braking, momentum causes a pitching forward of the bicycle and rider combination.
Additional ride performance characteristics are desirable for a DH bicycle. A bike with a short wheelbase, specifically the distance from the cranks to the rear axle is desirable for manoeuvrability in the air and to allow easier lifting of the front wheel over obstacles while pedaling. A vehicle that extends its wheelbase, again specifically when measured from cranks to rear axle, offers more stability on compression. A bike that will lower in the rear suspension on braking to counteract the previously discussed issue of a pitching forward of the rider and vehicle combination and thus developing a slacker head tube angle would serve to add stability in a braking scenario where bias was given to the rear brake. A bike that will rise or at the very least serve to counteract the unbalanced downward force of an abrupt pedal stroke to preserve or increase ground clearance during pedaling can be seen to be advantageous for both ground clearance and traction.
The prior art has not necessarily sought to accomplish the above ride characteristics while producing this rearward wheel path; however this invention serves that purpose as seen as beneficial to the implementation of a DH bike.